If the water coming out ur RO is 65 then adding a pre filter now is a bit late unless the tap water is somewhere above 500, u want about 90% reduction from ur RO.
If u just using one di bottle as well, I changed my resin a few weeks ago and ran water through it that was 53, it took a good 10 mins before it changed the tds to 0
I was thinking dodgy resin but has been fine since